1. Field of the Invention
The present invention relates to aqueous coating compositions curable at low temperatures, as well as multi-component composite coating compositions including pigmented or colored base coats overcoated with transparent or clear topcoats, providing good smoothness and appearance in automotive coating applications.
2. Background of the Invention
Over the past decade, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents that are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the automotive industry, without using organic solvents which contribute greatly to flow and leveling of a coating.
One of the major goals of the coatings industry is to minimize the use of organic solvents by formulating waterborne coating compositions which provide a smooth, high gloss appearance, as well as good physical properties including resistance to acid rain. Unfortunately, many waterborne coating compositions do not provide acceptable appearance, particularly when cured at lower temperatures, assuming low temperature cure is possible at all. Many automotive manufacturers are interested in coatings that cure at lower temperatures in order to reduce energy costs.
Another challenge to formulators of waterborne coatings is to provide good acid resistance while maintaining acceptable physical properties. Lack of humidity resistance or blushing is another problem facing waterborne coating formulators.
U.S. Pat. No. 4,350,809 to Fischer et al. discloses a process for the preparation of copolymers containing hydroxyl groups by free radical polymerization of at least two unsaturated copolymerizable monomers, at least one of which contains at least one carboxyl group, in the presence of at least one alkyl glycidyl ester of an aliphatic saturated monocarboxylic acid with a tertiary or quaternary α-carbon atom. The resulting polymers are advantageous because of their relatively low solution viscosities.
U.S. Pat. No. 5,663,265 to Epple et al. discloses low-viscosity copolymers useful in coating compositions. The copolymers contain hydroxyl and carboxyl groups and are obtained by free-radical polymerization of at least two olefinically unsaturated copolymerizable monomers of which at least one contains at least one carboxyl group and at least one is sterically hindered in the presence of one or more glycidyl esters of aliphatic saturated monocarboxylic acids having a tertiary or quaternary α-carbon atom. The copolymers are useful in clearcoat coating compositions, which demonstrate notable resistance to sulfuric acid and xylene, as well as a high degree of hardness.
U.S. Pat. No. 5,596,057 to Epple et al. discloses low viscosity copolymers prepared by bulk polymerization. The polymerization includes charging a material with functional groups that react with the carboxyl groups of the monomers and polymerizing at least two olefinically unsaturated copolymerizable monomers, at least one of which contains at least one carboxyl group and at least one of which is sterically hindered. The copolymers are useful in coating compositions.
U.S. Pat. No. 4,322,508 to Peng et al. discloses a thermosetting coating composition which includes a hydroxy functional component and a crosslinking agent capable of reacting with hydroxy functionality of the hydroxy functional component. The hydroxy functional component includes an oligoester formed by an esterification reaction between a carboxylic acid and an epoxide and a hydroxy functional copolymer. The hydroxy functional copolymer includes residues from one or more hydroxy functional monomers. The thermosetting compositions have a short shelf life, however, because the oligoester and hydroxy functional copolymer in the hydroxy functional component tend to separate over time.
It would be desirable to provide a coating composition which provides the benefits of including a combination of hydroxy functional polyesters and hydroxy functional acrylic copolymers, such that they do not tend to separate over time, providing a storage stable coating composition. The resultant coatings should have a balance of physical properties, including high gloss, hardness, impact resistance, flexibility, weatherability and chemical resistance.